Optoacoustic measuring arrangement and use thereof

ABSTRACT

The measuring arrangement contains a measuring cell and a reference cell ( 9, 10 ), respectively, and microphones ( 11  and  12 ) assigned to these cells, to which microphones an electronic evaluation circuit ( 7, 8 ) is connected and in which a subtraction of the signals of the microphones ( 11  and  12 ) takes place, as well as a radiation source ( 5 ) for applying a modulated signal to the measuring cell ( 9 ). The modulation frequency of the radiation source ( 5 ) coincides with the resonant frequency of the measuring cell ( 9 ), and the measuring cell and the reference cell ( 9  and  10 ) are open at at least one end to the gas and/or aerosol to be detected.  
     The measuring arrangement is used as smoke alarm, gas alarm, fire hazard alarm or as combined smoke and gas alarm, wherein each of the measuring cells ( 9 ) is exposed to a radiation of a wavelength at which a relevant substance to be detected is absorbent and an opto-acoustic effect is produced as a result.

DESCRIPTION

[0001] The present invention concerns an opto-acoustic measuringarrangement for the detection of gases and/or aerosols, having ameasuring cell and a reference cell, respectively, and microphonesassigned to these cells, to which microphones an electronic evaluationcircuit is connected, in which a subtraction of the signals of themicrophones takes place, and having a radiation source for applying amodulated signal to the measuring cell, wherein the modulation frequencyof the radiation source coincides with the resonant frequency of themeasuring cell.

[0002] With the opto-acoustic or photo-acoustic effect, an acousticpressure wave, whose magnitude is directly proportional to theconcentration of the relevant gas, is produced by the irradiation bymodulated light of a gas to be detected. The acoustic pressure wave isproduced because the gas absorbs the light radiation and heats up as aresult. This results in a thermal expansion and a periodic pressurefluctuation in accordance with the modulation of the light radiation.The two cells are usually termed the measuring cell and the referencecell, and the measuring arrangement is constructed so that the cells areeither separated from each other and the radiation passes through bothcells (C. F. Dewey, Jr.: Opto-acoustic Spectroscopy and Detection, [Y.H. Pao, ed.], Academic Press, New York, 1977, 47-77) or the cells areinterconnected and the radiation passes only through the measuring cell(G. Busse and D. Herboeck: Differential Helmholz resonator as anopto-acoustic detector, Applied Optics, Vol 18, No. 23, 3959).

[0003] With the detection of aerosols the behaviour is similar, as thesealso absorb the modulated radiation, whereby modulated heat and fromthis modulated pressure are produced as a result. Previously describedopto-acoustic sensors for the measurement of aerosols are mostlymono-sensors with only one measuring cell. If, for the aerosolmeasurement, sensors with two cells, so-called dual sensors having onemeasuring cell and one reference cell are proposed, then these areconstructed so that the reference cell is screened against aerosol. Thelatter is achieved by filtering the air before it reaches the referencecell. Reference is also made to the severe temperature-dependence of theresonant frequency, which requires correction of the signal magnitude.

[0004] When the dual principle. is employed, the detection sensitivityof opto-acoustic sensors for gases or aerosols is in the region of thatof optical smoke alarms. Since the opto-acoustic signals are produced byabsorption and not by radiation, both large and even the smallestaerosols were able to be detected to below the p range with theopto-acoustic principle, and light and dark types of smoke were able tobe measured to a more or less equal degree. Nevertheless, theopto-acoustic principle is not being used up to now for smoke detection,which is chiefly due to the additional outlay necessitated by the airfiltering and the correction of the signal magnitude.

[0005] An opto-acoustic measuring arrangement of the type stated at theoutset, whose costs are competitive with those of a scattered-lightdetector, shall now be specified by the invention.

[0006] This problem is solved according to the invention in that themeasuring cell and the reference cell are open at at least one end tothe gas and/or aerosol to be detected.

[0007] Since in the opto-acoustic measuring arrangement according to theinvention both cells are open to the gas and/or aerosol to be detected,filtering of the gas/aerosol to be investigated is not necessary.Normally the sensor signal is zero, and a signal which requires only arelatively simple electronic circuit for its processing is produced inthe measuring cell only in the presence of aerosol or a combustible gaswhich absorbs the radiation emitted by the radiation source.

[0008] A first preferred embodiment of the measuring arrangementaccording to the invention is characterised in that the electronicevaluation circuit contains a differential amplifier and aphase-sensitive rectifier.

[0009] A second preferred embodiment of the measuring arrangementaccording to the invention is characterised in that the wavelength ofthe radiation emitted by the radiation source is chosen so that it isabsorbed by the gas to be detected. A first photocell for monitoring theintensity of the radiation emitted by the radiation source is preferablydisposed within the range of the radiation source.

[0010] A third preferred embodiment of the measuring arrangementaccording to the invention is characterised in that in addition to themeasuring cell, a second photocell is disposed which, in the presence ofan aerosol, is exposed to the scattered radiation of the radiationsource caused by this aerosol.

[0011] A measuring arrangement constructed in this manner can detectboth an aerosol, that is to say smoke, and a gas, and is thereforeeminently suitable for use as a so-called dual-criteria alarm for smokeand gas. In practice it behaves in such a way that a specific aerosol isabsorbed in a specific wavelength range, the type of aerosol dependingon the combustible material. However, since the smoke from a fire nearlyalways contains mixtures of organic substances, such as wood, forexample, which are very absorbent in the entire infrared range and stillsufficiently absorbent in the visible light range, the choice ofwavelength for optimum aerosol detection is not critical.

[0012] If only smoke is to be detected, the second photocell is notrequired because in this case a wavelength can be selected, at which nocombustible gases are absorbent. In the detection of smoke and gas, thelateral photocell is always required when a gas whose absorption rangeis that of aerosol, is to be detected.

[0013] A fourth preferred embodiment of the measuring arrangementaccording to the invention is characterised in that the measuring cellis radiated by two radiation sources which are operated at differentfrequencies. This arrangement is suitable for the detection of smoke andtwo gases.

[0014] A further preferred embodiment of the measuring arrangementaccording to the invention is characterised in that two pairs ofmeasuring cells and reference cells, open at both ends, are provided,each of which has a different length and thus different resonantfrequencies, that a microphone is assigned to each reference cell pairand to each measuring cell pair, and that each measuring cell is exposedto a radiation source.

[0015] The measuring arrangement with the two pairs of measuring cellsand reference cells is suitable for the detection of smoke and twogases. By adding a further pair with a measuring cell and a referencecell, the detection range of the measuring arrangement can be extendedto a third gas.

[0016] The invention further concerns an application of said measuringarrangement as a smoke alarm. This application is characterised in thatthe measuring arrangement has a measuring cell which is exposed to aradiation of a wavelength at which the aerosol to be detected isabsorbent and an opto-acoustic effect is produced as a result.

[0017] The invention further concerns an application of said measuringarrangement as a fire hazard alarm. This application is characterized inthat the measuring arrangement has a measuring cell which is exposed toa radiation of a wavelength at which a combustible or explosivesubstance to be detected is absorbent and an opto-acoustic effect isproduced as a result.

[0018] The invention further concerns an application of said measuringarrangement as a combined smoke and gas alarm. This application ischaracterised in that the measuring arrangement has a measuring cellwhich is exposed to a radiation of a wavelength at which a combustibleor explosive substance to be detected is absorbent and an opto-acousticeffect is produced as a result, and that in addition to the measuringcell, a photocell is disposed so that it is exposed to scattered lightof the radiation, caused by an aerosol.

[0019] The invention further concerns an application of said measuringarrangement as a combined fire alarm and fire hazard alarm. Thisapplication is characterised in that the measuring arrangement has twomeasuring cells, of which one is exposed to radiation of a wavelength atwhich the aerosol or a combustible gas to be detected is absorbent, andof which the other is exposed to radiation of a wavelength at which acombustible or explosive substance to be detected is absorbent and anopto-acoustic effect is produced as a result.

[0020] The invention is explained in further detail below with the aidof exemplary embodiments and the drawings, in which:

[0021]FIG. 1 shows a schematic representation of a resonant,opto-acoustic dual sensor, open at one end for smoke and gas;

[0022]FIG. 2 shows a schematic representation of a resonant,opto-acoustic dual sensor, open at both ends for smoke and gas; and

[0023]FIG. 3 shows a development of the dual sensor of FIG. 2.

[0024] The opto-acoustic measuring arrangement illustrated in FIG. 1 isa resonant, dual sensor, open at one end, with a tubular measuring cell1 and a tubular reference cell 2 to each of which a microphone 3 and 4,respectively, is assigned. Furthermore, a radiation source 5, forexample an LED, is provided, which exposes the inner space of themeasuring cell 1 with radiation of a specific wavelength. In addition tothe radiation source 5, a first photocell 6 is disposed for monitoringthe intensity of the radiation emitted by the radiation source 5. Theoutputs of the two microphones 3 and 4 are fed to a differentialamplifier 7 in which the microphone signals are subtracted from eachother. The output signal of the differential amplifier 7 is fed to aphase-sensitive rectifier (lock-in) 8.

[0025] Tubes open at one end, with a length l have a resonant frequencyv_(k), which is given by${{\upsilon \quad k} = {\frac{{2k} + 1}{4l}c}},$

[0026] (k=0, 1, 2, 3, . . . ; c=velocity of sound in air)

[0027] With a length l of 2 cm, this gives a resonant frequency v₀=4.1kHz; with a tube open at both ends this resonant frequency is doubled.Standing waves therefore occur in the tube, wherein in the case of thetube open at one end, a pressure antinode (=motion node) occurs at theclosed end and a pressure node (=motion antinode) occurs at the openend. In the tube open at both ends, the pressure antinode is located atthe centre of the tube and a motion antinode at each open end.

[0028] The radiation source 5 emits modulated radiation into themeasuring cell 1, the modulation frequency of the radiation source 5coinciding with the resonant frequency of the measuring cell. If themeasuring cell 1 contains an aerosol, then this absorbs the modulatedradiation, thereby producing modulated heat. The modulated heat producesmodulated pressure and thus sound at the frequency of the resonantfrequency of the measuring cell 1, as a result of which the air columnin the measuring cell is excited into oscillation. The same applies inthe presence of a gas in the measuring cell 1. The microphone 3, whichis located at the position of a pressure antinode. Of the standing wave,measures the oscillations (=sound) in the tube. As soon as themicrophone 3 measures a sound which coincides with the resonantfrequency of the measuring cell 1, there is an aerosol and/or a gas inthe measuring cell 1.

[0029] In contrast to a scattered-light smoke alarm, the illustratedmeasuring arrangement responds equally well to dark and light aerosols:dark aerosols produce a large signal because when the radiation of theradiation source 5 initially strikes a particle much radiation power isabsorbed. And light aerosols likewise produce a large signal, since theradiation at the light particles is reflected many times and in total isabsorbed to a great extent. Moreover, the opto-acoustic sensor respondsboth to large aerosols and to very small ones below the p range, sincethe opto-acoustic signals are generated by absorption and not byscattering.

[0030] The microphone 3 measures not only the resonant oscillations inthe measuring cell 1, but also all noises in the room, which can lead tointerference. This interference is eliminated by the reference cell 2and the microphone 4. Since the reference cell 2 is not exposed to theradiation of a radiation source, the microphone 4 also cannot measureany oscillations produced by a radiation source, but exclusivelymeasures the noise in the room. The signals of the reference microphone4 are subtracted in the differential amplifier 8 from the signals of themeasuring microphone 3, thus eliminating the room noise. Vibrations,which have an equal effect on both microphones, are likewise eliminated.The two cells, measuring cell and reference cell, can also be open atboth ends.

[0031] Such an arrangement with a measuring cell 9, open at both ends, areference cell 10, open at both ends, a measuring microphone 11 and areference microphone 12, is illustrated in FIG. 2. A second photocell13, which is disposed in the region between the radiation source 5 andthe measuring cell 1, is also shown in FIG. 2. The position of thesecond photocell 13 is chosen so that in the presence of particles inthe area between radiation source 5 and measuring cell 1, a portion ofthe scattered light of the radiation of the radiation source 5 producedby these particles falls on the photocell 13. The second photocell 13enables a distinction to be made between aerosol and gas. If both themeasuring cell 1 and the second photocell 13 deliver a signal, then anaerosol is present. If only the measuring cell 1 delivers a signal, theneither a gas or a very small and therefore non-scattering aerosol ispresent.

[0032] If the suppression of room noise and vibrations can be dispensedwith, in principle a measuring arrangement without reference cell 2 andthe microphone 4 assigned to the former could be adequate. If in such anarrangement the wavelength of the radiation source 5 is placed on theCO₂ line, then the measuring arrangement will measure very sensitivelythe concentration of the combustion gas CO₂ on the one hand and theconcentration of aerosol on the other.

[0033] The measuring arrangement illustrated in FIGS. 1 and 2 can bedesigned as a gas alarm, smoke (aerosol) alarm, as a combined gas andsmoke alarm and it can be used in these various forms as a fire alarm oras a fire hazard alarm. A fire alarm detects smoke and/or combustiongases, or generally, substances which characterise a fire. A fire hazardalarm detects, on the one hand, an existing fire by detecting an aerosolor substances occurring in a fire. On the other hand it detects toxicsubstances occurring in a fire and it recognises the danger of apossible fire or a possible explosion by detecting the presence ofcombustible substance in the air.

[0034] Substances which characterise a fire are, in particular, thefollowing: CO₂, CO, NO, NO₂, SO₂, NH₃, HCl, HF, HCN, amine and amide,compounds containing hydrocarbons, C, O and H; aerosols. Combustiblesubstances are generally hydrocarbons, particularly CH₄, C₂H₆, C₃H₈,C₄H₁₀, C₂H₂, C₂H₄, as well as general solvents, alcohol, ether, ketone,aldehyde, amine and amide, in particular methanol, ethanol, n-propanol,diethylether, acetone. Other combustible substances which a fire hazardalarm should detect are compounds containing C, O and H, and carboxylicacids. Toxic substance are CO₂, CO, NO, NO₂, SO₂, NH₃, HCl, HF, HCN,H₂S, nitriles, phosphoric ester, mercaptans, halogenated compounds.

[0035] Since the velocity of sound in air is temperature-dependent andcan vary by up to 30% in the temperature range of a fire alarm from −20°C. to +70° C., the resonant frequency can also change accordingly. Watervapour also influences the velocity of sound and thus the resonantfrequency. In order to eliminate these influences, the rough range ofthe resonant frequency and the possible-additional expansion of thefrequency range by varying water vapour content in the air, can becalculated with a temperature measurement and the modulation frequencyof the radiation source varied (swept) in this range.

[0036] A further possible disturbance consists in frequencies in theroom, which coincide with the resonant frequency. Such frequenciesexcite both cells into oscillation, but cannot be completely subtractedto zero by the differential circuit, because, due to the distance fromthe centre of the measuring cell 1, 9 to the centre of the referencecell 2, 10, they strike the cells with a time shift and excite thesecells into oscillation which has a small phase shift. This phase shiftcan be minimised by a lowest possible resonant frequency, because theinterfering audio frequencies then have a large acoustic wavelength andthe phase shift becomes small. Or, the signal of the reference cell 2,10 can be measured separately and when a signal, which in fact can begenerated only from outside, impinges upon the reference cell, the alarmthreshold of the measuring arrangement can be increased.

[0037] Further potential disturbance variables are different lengths ofthe cells. These disturbance variables can be eliminated by measuringthe resonant frequency of one of the two cells and mechanically varyingthe length of the other cell accordingly. The resonant frequency of thereference cell can also be measured and the radiation source 5positioned so that its position influences the resonant frequency of themeasuring cell and brings it into coincidence with the reference cell.

[0038] As a further check, monitoring of the microphone sensitivity bymeans of the zero signals produced by the radiation source in the wallof the measuring cell 1, 9, which occur under all environmentalconditions, is recommended.

[0039] The arrangement illustrated in FIGS. 1 and 2 for themeasurement/detection of smoke and one gas can be expanded by anadditional pair of cells for the measurement/detection of a further gas.According to FIG. 3, an additional measuring cell 14, an additionalreference cell 15 and an additional radiation source 16 are provided,where the measuring cell 9 measures aerosol and a first gas and themeasuring cell 14 measures a second gas, for example. The two measuringcells 9 and 14 and, correspondingly, also the two reference cells 10 and15 have different lengths and therefore also different resonantfrequencies, and the two measuring cells are exposed to radiation fromthe radiation sources 5 and 16, respectively, at different wavelengths.The two different resonant frequencies can be measured with just onemeasuring microphone 11. Likewise, only one reference microphone 12 andonly one single photocell 6 are required for monitoring the emission ofboth radiation sources 5 and 16.

[0040] The measuring cells and reference cells can have the followingdimensions, for example: Measuring cell 9, reference cell 10: lengtheach 2 cm, resonant frequency each 8.2 kHz Measuring cell 14, referencecell 15: length each 2.2 cm, resonant frequency each 7.6 kHz

[0041] Accordingly, the modulation frequency of the radiation source 5is 8.2 kHz and that of the radiation source 16 is 7.6 kHz. LEDs are usedas radiation sources.

[0042] The additional outlay for the detection of a second gas is thusonly the costs of the second cell pair and for the second radiationsource. It is quite obvious that an extension for the detection of athird gas requires only a further cell pair and a further radiationsource.

[0043] Instead of two pairs of measuring and reference cells (9, 10; 14,15), of different length, which are simultaneously irradiated by tworadiation sources 5 and 16, in the arrangement of FIG. 2 the measuringcell 9 of one cell pair can be simultaneously irradiated by tworadiation sources 5 and 16 and these can be operated at differencefrequencies; for example, the radiation source 5 at the fundamentalfrequency and the radiation source 16 at the first harmonic.Consequently, compared to the arrangement of FIG. 3, only half thenumber of cells and microphones are needed and corresponding costs aresaved.

[0044] Apart from the resonant opto-acoustic dual sensors, open at oneend or both ends, non-resonant, closed dual sensors are also known (seefor example EP-A-0 855 592), which can likewise be constructed so thatthe detection of aerosols and gases is possible with them. As can beseen in EP-A-0 855 592, these opto-acoustic dual sensors contain ameasuring cell and a reference cell, each of which is sealed against theenvironment by a diaphragm, and a radiation source. Gas can permeate thecell through the diaphragm. A measuring microphone and a referencemicrophone are provided, the reference microphone being screened againstopto-acoustic signals of the gas/aerosol to be detected. So that aerosolparticles can permeate the cells, the pore size of the diaphragms isincreased accordingly.

[0045] But as a result, the diaphragms for frequencies below 500 Hz areacoustically soft, pressure build-up in the cell is no longer possibleand the sensitivity is seriously reduced. By increasing the modulationfrequency to a few kilohertz, the diaphragms again become acousticallyhard and the sensitivity no longer decreases. Any jamming of thediaphragms can be monitored by measuring the reference signalseparately, which gives a base noise level, and the sensitivitycorrected with the aid of this base level. If the wavelength of theradiation sources is positioned on the CO₂ line, for example, then themeasuring arrangement will measure very sensitively the concentration ofthe CO₂ combustion gas. On the other hand, however, the concentration ofaerosol is very sensitively measured because cellulose and carbonisedcellulose particles are strongly absorbent in the entire infrared range.The volume per cell is approximately 2 times 2 times 2 cm³.

1. Opto-acoustic measuring arrangement for the detection of gases and/oraerosols, having a measuring cell and a reference cell (1, 9, 14 and 2,10, 15), respectively, and microphones (3, 11 and 4, 12) assigned tothese cells, to which microphones an electronic evaluation circuit (7,8) is connected, in which a subtraction of the signals of themicrophones (3, 11 and 4, 12) takes place, and having a radiation source(5, 16) for applying a modulated signal to the measuring cell (1, 9,14), wherein the modulation frequency of the radiation source (5, 16)coincides with the resonant frequency of the measuring cell (1, 9, 14),characterised in that the measuring cell and the reference cell (1, 9,14 and 2, 10, 15) are open at at least one end to the gas and/or aerosolto be detected.
 2. Measuring arrangement according to claim 1,characterised in that the electronic evaluation circuit contains adifferential amplifier (7) and a phase-sensitive rectifier (8). 3.Measuring arrangement according to claim 1 or 2, characterised in thatthe wavelength of radiation emitted by the radiation source (5, 16) ischosen so that it is absorbed by a gas to be detected.
 4. Measuringarrangement according to claim 3, characterised in that a firstphotocell (6) for monitoring the intensity of the radiation emitted bythe radiation source (5, 16) is disposed in the region of the radiationsource (5, 16).
 5. Measuring arrangement according to claim 3,characterised in that in addition to the measuring cell (9, 14) a secondphotocell (13) is disposed, which, in the presence of an aerosol, isexposed to the scattered radiation of the radiation source (5, 16)caused by this aerosol.
 6. Measuring arrangement according to one ofclaims 1 to 5, characterised in that the measuring cell (9) is exposedto two radiation sources (5, 16), which are operated at differentfrequencies.
 7. Measuring arrangement according to claim 6,characterised in that one of the radiation sources (5) is operated atthe fundamental frequency and the other is operated at the firstharmonic.
 8. Measuring arrangement according to claim 4 or 5,characterised in that two pairs of measuring cells and reference cells(9, 14; 10, 15), open at both ends, are provided, each of which has adifferent length and thus different resonant frequencies, that amicrophone (11 and 12) is assigned to each reference cell and to eachmeasuring cell pair (9, 14 and 10, 15), and that each measuring cell (9,14) is exposed to a radiation source (5 and 16).
 9. Measuringarrangement according to one of claims 1 to 8, characterised in that asensor for measuring the ambient temperature is provided, and that anadjustment of the modulation frequency of the radiation source (5, 16)to a frequency range corresponding to the measured ambient temperature,and a time-shift of the modulation frequency within this frequencyrange, takes place.
 10. Application of the measuring arrangementaccording to one of claims 1 to 9 as a smoke alarm, characterised inthat the measuring arrangement has a measuring cell (1, 9, 14), which isexposed to a radiation of a wavelength at which the aerosol to bedetected is absorbent, and an opto-acoustic effect is produced as aresult.
 11. Application of the measuring arrangement according to one ofclaims 1 to 9 as a fire hazard alarm, characterised in that themeasuring arrangement has a measuring cell (1, 9, 14), which is exposedto a radiation of a wavelength at which a combustible or explosivesubstance to be detected is absorbent and an opto-acoustic effect isproduced as a result.
 12. Application according to claim 11,characterised in that the combustible or explosive substance to bedetected is formed by one or more of the following substance:hydrocarbons, particularly CH₄, C₂H₆, C₃H₈, C₄H₂O, C₂H₂, C₂H₄, as wellas general solvents, alcohol, ether, ketone, aldehyde, amine and amide,in particular methanol, ethanol, n-propanol, diethylether, acetone,compounds containing C, O and H, and carboxylic acids.
 13. Applicationaccording to claim 11, characterised in that the measuring arrangementhas a measuring cell (1, 9, 14), which is exposed to a radiation of awavelength at which a toxic substance to be detected is absorbent and anopto-acoustic effect is produced as a result.
 14. Application accordingto claim 13, characterised in that the toxic substance to be detected isformed by one or more of the following substances: CO₂, CO, NO, NO₂,SO₂, NH₃, HCl, HF, HCN, H₂S, nitrites, phosphoric ester, mercaptan,halogenated compounds.
 15. Application of the measuring arrangementaccording to one of claims 1 to 9, as a combined smoke and gas alarm,characterised in that the measuring arrangement has a measuring cell (9,14), which is exposed to a radiation of a wavelength at which acombustible or explosive substance to be detected is absorbent and anopto-acoustic effect is produced as a result, and that in addition tothe measuring cell (9, 14), a photocell (13) is disposed so that it isexposed to scattered light of the radiation, caused by an aerosol.